With increasing development of digital technology, panel displays become essential components of many electrical appliances such as notebooks, mobile phones, information appliances (IA) and personal digital assistants (PDAs). Since the typical liquid crystal display (LCD) needs backlight and is complicated to be fabricated, alternative displays are further developed. Recently, a display by means of organic light-emitting diodes (OLEDs) has been developed due to its self-light-emitting and easily manufactured features. In addition, the OLED display has advantages of wider viewing angle, low cost, reduced thickness and flexible operational temperature. The OLEDs can be used in pixel units of an active matrix electron luminescent display to emit light, and the OLED display is expected to substitute for the LCD in the near future. The OLED pixels are generally driven in either a voltage-driving manner, as shown in FIG. 1, or a current-driving manner, as shown in FIG. 2, which will be described hereinafter.
Please refer to FIG. 1, in which a conventional pixel driving circuit of an OLED display is shown. Each of the pixel units comprises an organic light-emitting diode OLED, two transistors M1˜M2 and a capacitor Cs. The gate electrode of the transistor M1 is coupled to a gate line 10, and the other two electrodes of the transistor M1 are coupled to a data line 20 and the gate electrode of the transistor M2, respectively. The source and drain electrodes of the transistor M2 are coupled to a source voltage Vdd and the P electrode of the organic light-emitting diode OLED. The N electrode of the organic light-emitting diode OLED is coupled to a ground voltage GND. The capacitor Cs is coupled between the source electrode and gate electrode of the transistor M2.
During operation of the gate line 10, the transistor M1 is switched on. Meanwhile, via the data line 20, a driving voltage is inputted and stored in the capacitor Cs. The driving voltage can also bias the transistor M2 to result in a constant current Id passing through the organic light-emitting diode OLED. The organic light-emitting diode OLED emits light accordingly.
For a purpose of forming the active matrix and its peripheral circuit on the same substrate, a so-called low-temperature polysilicon thin film transistor (LTPS-TFT) technology was developed with improved electrical properties of TFTs and other benefits. However, since the threshold voltage and mobility of such LTPS-TFT vary with manufacturing processes to a certain extent, some problems may occur. For example, under a constant voltage applied to the capacitor Cs, the resulting intensity of current passing through the organic light-emitting diode OLED may be different for the LTPS-TFT manufactured by different processes. The light intensity emitted by the OLED cannot be well expected.
FIG. 2 illustrates another conventional driving circuit for driving an OLED pixel. Each of the pixel units comprises an organic light-emitting diode OLED, four transistors M1˜M4 and a capacitor Cs. The gate electrode of the transistor M1 is coupled to a first scan line 30, and the other two electrodes of the transistor M1 are coupled to a data line 50 and the drain electrode of the transistor M3, respectively. The gate electrode of the transistor M2 is coupled to the first scan line 30, and the other two electrodes of the transistor M2 are coupled to the data line 50 and the gate electrode of the transistor M3, respectively. The source and drain electrodes of the transistor M3 are coupled to a source voltage Vdd and the drain electrode of the transistor M4, respectively. The gate and drain electrodes of the transistor M4 are coupled to a second scan line 40 and the P electrode of the organic light-emitting diode OLED, respectively. The N electrode of the organic light-emitting diode OLED is coupled to a ground voltage GND. The capacitor Cs is coupled between the source electrode and gate electrode of the transistor M3.
The circuit of FIG. 2 can be operated in either a memorizing or an emission state, which are controlled by the first scan line 30 and the second scan line 40, respectively. The first scan line 30 and the second scan line 40 use the same clock signal. When the clock signal is at a high level, the first scan line 30 operates and thus the transistors M1 and M2 are switched on. Whereas, when the clock signal is at a low level, the second scan line 40 operates and thus the transistor M4 is switched on.
When the circuit is operated in the memorizing state, the first scan line 30 works to switch on the transistors M1 and M2, but the second scan line 40 suspends operation such that the transistor M4 is switched off. At this time, a current from the voltage source Vdd will charge the capacitor Cs to generate voltage. The voltage applied to the capacitor Cs can bias the transistor M3 to result in a driving current Id1 passing through the transistors M3 and M1 to the data line 50. Meanwhile, no driving current passes through the transistor M4.
When the circuit is operated in the emission state, the first scan line 30 suspends operation such that the transistors M1 and M2 are closed, but the second scan line 40 works to switch on the transistor M4. Therefore, the driving current Id1 is zero. At this time, the voltage applied to the capacitor Cs will bias the transistor M3 to result in a driving current Id2 passing through the organic light-emitting diode OLED. The organic light-emitting diode OLED emits light accordingly.
The deviations of threshold voltage and mobility, which are caused in the driving circuit of FIG. 1, can be compensated by using the driving circuit of FIG. 2. However, since the equivalent impedance at the drain electrode of the transistor M3, i.e. the node a, in the memorizing state and in the emission state are different, the driving currents Id1 and Id2 are different even when an identical biased voltage is applied. As can be seen in FIG. 3, when the transistor M3 is biased by various biased voltages VCs1˜VCs10, different quantities of driving currents Id1 and Id2 are observed in the memorizing and the emission states, respectively.